Rare earth element-doped silicon oxide film electroluminescence device

ABSTRACT

A method is provided for forming a rare earth (RE) element-doped silicon (Si) oxide film with nanocrystalline (nc) Si particles. The method comprises: providing a first target of Si, embedded with a first rare earth element; providing a second target of Si; co-sputtering the first and second targets; forming a Si-rich Si oxide (SRSO) film on a substrate, doped with the first rare earth element; and, annealing the rare earth element-doped SRSO film. The first target is doped with a rare earth element such as erbium (Er), ytterbium (Yb), cerium (Ce), praseodymium (Pr), or terbium (Tb). The sputtering power is in the range of about 75 to 300 watts (W). Different sputtering powers are applied to the two targets. Also, deposition can be controlled by varying the effective areas of the two targets. For example, one of the targets can be partially covered.

RELATED APPLICATIONS

The application is a continuation-in-part of a pending applicationentitled HIGH-LUMINESCENCE SILICON ELECTROLUMINESCENCE DEVICE, TingkaiLi et al., Ser. No. 11/066,713, filed on Feb. 24, 2005, which isincorporated herein by reference.

The application is a continuation-in-part of a pending applicationentitled WIDE WAVELENGTH RANGE SILICON ELECTROLUMINESCENCE DEVICE,Tingkai Li et al., Ser. No. 11/058,505, filed on Feb. 14, 2005, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to integrated circuit (IC) fabricationand, more particularly, to a sputter deposition procedure for making arare earth element-doped silicon-rich silicon oxide (SRSO) film withnanocrystalline (nc) Si particles, for use in electroluminescence (EL)applications.

2. Description of the Related Art

The observation of visible luminescence at room temperature, emanatingfrom porous silicon (Si), has spurred a tremendous amount of researchinto using nano-sized Si to develop a Si-based light source. One widelyused method of fabricating nanocluster Si (nc-Si) is to precipitate thenc-Si out of SiOx (x<2), producing a film using chemical vapordeposition (CVD), radio frequency (RF)-sputtering, and Si implantation,which is often called silicon-rich silicon oxide (SRSO). Erimplantation, creating Er-doped nanocrystal Si, is also used in Si basedlight sources. However, state-of-the-art implantation processes have notbeen able to distribute the dopant uniformly, which may be important forhigh-efficiency light emission. Ion implantation also increases costs.Interface engineering may also be important for the device performance,but it is very difficult to achieve using ion implantation. All thesedrawbacks limit future device applications.

Other work (Castagna et al., “High Efficiency Light Emission Devices inSilicon”) describes a silicon-based light source consisting of a MOSstructure with nc-Si particles and Er implanted in a thin oxide layer.After annealing at 800° C. for 30 minutes under nitrogen flux, thedevice shows 10% external quantum efficiency at room temperature, whichis comparable to that of light emitting diodes using III-Vsemiconductors. However, the stability of the device is poor. Anotherdevice structure consists of a 750 Å thick silicon-rich oxide (SRO) gatedielectric layer doped with rare earth ions (Er, Tb, Yb, Pr, Ce) via ionimplantation. After similar annealing, the device shows much more stableproperties but the efficiency drops off to 0.2%

Undoped silicon nano particles possess a wide wavelength distribution inits light emission spectrum, due to its particle size distribution. Onthe other hand, RE doped SRSO emits light in discrete wavelengthscorrespondent to the intra 4f transitions of the RE atoms. For example,the main emission wavelengths for terbium, ytterbium, and erbium-dopedSRSO are located at the wavelengths of 550 nm, 983 nm, and 1540 nmrespectively. The monochromaticity of the RE-related light emission fromdoped silicon nano particles provides much better control over thewavelength, giving it wider application in optical communications.

To fabricate doped silicon-rich oxide, RE ion implantation haspreviously been explored. Although ion implantation provides for purityand flexibility, it is expensive and the dosage that can be implanted islimited. Dopant concentration in any particular depth direction isdifficult to control and the concentration of dopant is not uniform.

SUMMARY OF THE INVENTION

The present invention provides a method of depositing RE-doped SRSO by asputtering process. The doped SRSO film, in turn, can be annealed into afilm that contains actively doped silicon nano particles imbedded insilicon oxide matrix for electroluminescence (EL) applications. In oneaspect for example, terbium (Tb)-doped SRSO is deposited in Edwards 360reactive DC sputtering system by using a specially designed RE-embeddedSi target. The annealed film shows very strong Tb-relatedphotoluminescence (PL) signals, making it useful in EL deviceapplications.

Accordingly, a method is provided for forming a rare earth (RE)element-doped silicon (Si) oxide film with nanocrystalline (nc) Siparticles. The method comprises: providing a first target of Si,embedded with a first rare earth element; providing a second target ofSi; co-sputtering the first and second targets; forming a Si-rich Sioxide (SRSO) film on a substrate, doped with the first rare earthelement; and, annealing the rare earth element-doped SRSO film. Thefirst target is doped with a rare earth element such as erbium (Er),ytterbium (Yb), cerium (Ce), praseodymium (Pr), or terbium (Tb).

The sputtering power is in the range of about 75 to 300 watts (W). Inone aspect, different sputtering powers are applied to the two targets.In another aspect, deposition is controlled by varying the effectiveareas provided by the two targets. For example, one of the targets canbe partially covered. This control creates doping profiles that are notavailable using ion implantation. For example, a uniformly doped SRSOfilm can be formed in a single co-sputtering process. As anotherexample, a first thickness of SRSO film can be formed having a first REfirst doping concentration, and a second thickness of SRSO film can beformed overlying the first thickness, having a first RE second dopingconcentration, different from the first concentration.

Additional details of the above-described process and EL devicesfabricated using the above-mentioned process are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a silicon-rich silicon oxide(SRSO) film.

FIG. 2 is a partial cross-sectional view of a variation of the SRSO filmof FIG. 1

FIG. 3 is a partial cross-sectional view of an electroluminescence (EL)device.

FIG. 4 is a partial cross-sectional view of a variation of the EL deviceof FIG. 3.

FIG. 5 is a graph depicting EL measurements of an exemplary EL device.

FIG. 6 is a plot depicting the realtionship between PL, sputteringpower, and annealing conditions.

FIG. 7 is a flowchart illustrating a method for forming a REelement-doped Si oxide film with nc Si particles.

FIG. 8 is a schematic block diagram of a DC sputtering system using a Sitarget and a RE-doped Si target.

DETAILED DESCRIPTION

FIG. 1 is a partial cross-sectional view of a silicon-rich silicon oxide(SRSO) film. The SRSO film 100 comprises a first thickness 102 dopedwith a first concentration of a rare earth (RE) element. A secondthickness 104, overlies the first thickness 102, and is doped with asecond concentration of the RE element. In one aspect, the firstconcentration of RE dopant is greater than the second concentration. Inanother aspect, the second concentration of RE dopant is greater thanthe first. The film of FIG. 1 is intended to depict a simple exemplaryRE doping profile that can be obtained using a two-target sputteringprocess to deposit the RE-doped SRSO film 100. Other, more complicated,profiles may be created using the same basic methodology.

As in all the SRSO films described below, SRSO film 100 is primarilysilicon dioxide, with extra Si. After annealing, the Si atomsagglomerate together to form Si nano particles imbedded in a siliconoxide matrix. The silicon richness can be represented by refractiveindex of the film n. For pure SiO2, n=1.46. For SRSO film 100, n variesfrom 1.5 to 2.2. Further, the SRSO film 100 is doped with an REconcentration in the range of 2 to 10%.

FIG. 2 is a partial cross-sectional view of a variation of the SRSO filmof FIG. 1. The SRSO film 200 comprises a first thickness 202 doped witha first RE element. A second thickness 204 overlies the first thickness202, and is doped with a second RE element. The film of FIG. 2 isintended to depict a simple exemplary RE doping profile that can beobtained using a three-target sputtering process to deposit the RE-dopedSRSO film 200. More than three targets can be used to create morecomplex profiles.

FIG. 3 is a partial cross-sectional view of an electroluminescence (EL)device. The EL device 300 comprises a bottom substrate 302. Asilicon-rich silicon oxide (SRSO) film 304 includes a first thickness306 overlying the substrate 302, doped with a first concentration of arare earth (RE) element. A second thickness 308 of SRSO overlies thefirst thickness, and is doped with a second concentration of the REelement. In one aspect, the first concentration of RE dopant is greaterthan the second concentration. In another aspect, the secondconcentration of RE dopant is greater than the first. A top electrode(TE) 310 overlies the SRSO film 304.

In one aspect, the substrate 302 is either an n-type or p-type Sisubstrate. In other aspects, the substrate 302 or the top electrode 310can be a transparent material such as ITO, ZnO:Al, or Au. Othermaterials that can be used for the substrate and top electrode includealuminum (Al), zinc oxide (ZnO), chromium (Cr), Pt, Ir, AlCu, Ag, YBCO,RuO₂, and La_(1-x)Sr_(x)COO₃. The device of FIG. 3 is intended to depicta simple exemplary EL device that can be obtained using a two-targetsputtering process to deposit the RE-doped SRSO film 304.

For light generated using Er doping, the light at IR wavelengths can bedetected through a silicon substrate 302. In this case, the topelectrode can be opaque. For the light generated via Tb emission, thewavelength is around 550 nm, in the visible range. The top electrode inthis case must necessarily be a transparent for the light to bedetected, as visible-range light cannot be detected through a Sisubstrate.

FIG. 4 is a partial cross-sectional view of a variation of the EL deviceof FIG. 3. The EL device 400 comprises a bottom substrate 402 and a SRSOfilm 404. A first thickness 406 of SRSO overlies the substrate 402, andis doped with a first rare earth (RE) element. A second thickness 408 ofSRSO overlies the first thickness 406, and is doped with a second REelement. For example, the first RE can be Er and the second RE can beTb. A top electrode 410 overlies the SRSO film 404.

Functional Description

By using a specially designed RE-imbedded target with another, puresilicon target, RE-doped SRSO film can be deposited in differentconcentration profiles, to fabricate SRSO films with varied dopantconcentrations. As an example, the fabrication of a Tb-doped siliconnano-particle SRSO thin film is presented below. The deposition andannealing conditions are listed in Table 1. The sputtering power can bechanged to alter the silicon richness, while maintaining the Tb/Siratio. PL measurements associated with these samples, with a variety ofannealing conditions (from as-deposited to 1000° C.) are also presented.TABLE 1 Deposition and post annealing conditions for Tb-doped SRSO filmsSputtering Annealing power Pressure Temperature Gas temperature 75-300 W7-8 mtorr 225° C. 15% 900-1100° C. O₂/Ar

FIG. 5 is a graph depicting EL measurements of an exemplary EL device.The device tested has a Si substrate, covered with a 2.8 nm layer ofsilicon dioxide. A Tb-doped SRSO film overlies the silicon dioxide, andan ITO top electrode covers the SRSO film. The SRSO was sputtered at apower level of 300 W, annealed in an oxygen atmosphere at 950° C., for 4minutes. The emissions at 550 nm prove that Tb is incorporated into thefilm. Quite different from EL emission from silicon nano-particles,RE-related EL emission have discrete wavelengths that correspondent tothe intra-4f transition in the RE atoms. The graph shows thatpost-deposition annealing does not shift the emission wavelengths, whichis another proof of RE involvement. Conventional silicon nano particlefilms, without RE dopants, normally show a shift in emission wavelengthas a result of varied annealing conditions.

FIG. 6 is a plot depicting the realtionship between PL, sputteringpower, and annealing conditions. The peak PL intensity (at 544 nm)changes with annealing conditions and sputtering power. A systematicshift of the maximum PL intensity is associated with a higher sputteringpower when the annealing temperature is increased. The maximum PL occursat an annealing temperature of 1000° C. using MRL equipment, in N₂, withan annealing time of 10 minutes (m), when the sputtering power is 125 W.At a sputtering power of 75 W, the composition of the film is basicallySiO₂ with no extra silicon. The PL intensity decreases with an increasein annealing temperature. These relationships show the versatility ofthe RE-doped sputtering method, and point to new silicon nano-particlebased EL device applications.

In one simple aspect, a RE-doped SRSO film with different dopingconcentrations can be deposited by using different sputtering power ontwo targets, and/or by partially covering one of the targets. In anotheraspect, the doping concentration can be varied across the SRSO filmthickness, by varying the sputtering power during the deposition, or byvarying the exposed area of either one of the target during thedeposition. In this manner, the doping concentration can be manipulatedto achieve dopant profile engineering and/or interface engineering.

FIG. 8 is a schematic block diagram of a DC sputtering system using a Sitarget and a RE-doped Si target. The ions are supplied by a plasma thatis induced in the sputtering equipment. A variety of techniques are usedto modify the plasma properties, especially ion density, to achieve thedesired sputtering conditions. The target can be biased with a directcurrent (DC) voltage (DC sputtering). Alternating radio frequency (RF)current can also be used to bias the target. Magnetron sputteringsystems use magnetic fields to control and confine ion flow. While DCsputtering equipment is the simplest and cheapest to use, the presentinvention sputtering process can be enabled with any sputtering process.

Although the sputtered atoms are ejected from the target in a gas phase,they condense back into a solid phase upon colliding a substrate, whichresults in deposition of a thin film of sputtered material.

FIG. 7 is a flowchart illustrating a method for forming a REelement-doped Si oxide film with nc Si particles. Although the method isdepicted as a sequence of numbered steps for clarity, no order should beinferred from the numbering unless explicitly stated. It should beunderstood that some of these steps may be skipped, performed inparallel, or performed without the requirement of maintaining a strictorder of sequence. The method starts at Step 700.

Step 702 provides a first target of Si, embedded with a first rare earthelement. Some exemplary rare earth elements include erbium (Er),ytterbium (Yb), cerium (Ce), praseodymium (Pr), and terbium (Tb).However, the method is not limited to merely these examples. Step 704provides a second target of Si. Step 706 provides a substrate. Forexample, the substrate can be doped Si, or doped Si with an overlyinglayer of silicon oxide having a thickness in the range of about 1 to 10nm. Step 708 co-sputters the first and second targets. Step 710 forms aSRSO film on the substrate, doped with the first rare earth element.Step 712 anneals the rare earth element-doped SRSO film.

In one aspect, Step 708 co-sputters the first and second targets using asputtering power in the range of about 75 to 300 W. In another aspect,Step 708 uses an environmental pressure in the range of about 7 to 8milli-Torr. Typically, the substrate is heated to a temperature of about225° C., and the first and second targets are co-sputtered using an Aratmosphere, with about 15% oxygen.

In one aspect, equal sputtering power is applied to the two targets.Alternately, Step 708 sputters the first target at a first sputteringpower and the second target at a second sputtering power, different thanthe first sputtering power. A similar effect can be achieved byproviding a first target with a first effective area in Step 702, andproviding a second target in Step 704 with a second effective area,different than the first area. For example, one target can be made witha smaller surface area or partially covered.

Using one of the above-mentioned techniques, Step 710 may form a SRSOfilm with a RE doping profile that includes a first thickness of SRSOfilm having a first RE first doping concentration. A second thickness ofSRSO film overlies the first thickness, having a first RE second dopingconcentration, different from the first concentration.

In a different aspect, Step 712 anneals at a temperature in the range ofabout 900 to 1100° C., for a duration in the range of 10 to 30 minutes.The annealing is done in an atmosphere that may include N₂, inert gases,water vapor, oxygen, or a combination of the above-mentioned elements.The atmosphere chosen is often dependent upon the degree of oxidationdesired.

As noted in the explanation of FIG. 6 above, in one aspect Step 708decreases the sputtering power. Then, Step 712 decreases the annealingtemperature and increases the annealing time, in response to decreasingthe sputtering power. Alternately, if Step 708 increases the sputteringpower, then typically, Step 712 increases the annealing temperature anddecreases the annealing time, as a response to increasing the sputteringpower.

In one aspect, Step 710 forms a uniformly doped SRSO film as a result ofa single co-sputtering process (Step 708). This is a result that cannotbe achieved using ion implantation.

In one variation of the method, an additional step, Step 705, provides athird target of Si, embedded with a second rare earth element. Then,Step 708 co-sputters the first, second, and third targets, and Step 710forms a SRSO film doped with the first and second rare earth elements.For example, Step 710 may form a SRSO film with a RE doping profile thatincludes a first thickness of SRSO film doped with the first RE firstelement and a second thickness of SRSO film, overlying the firstthickness, doped with the second RE element. It should be noted thatStep 708 does not necessarily apply power to all three targetssimultaneously. For example, the above-mentioned doping profile isachieved by initially co-sputtering just the first and second targets,and later, co-sputtering just the second and third targets.

A sputter deposition method has been provided for the fabrication of arare earth element-doped SRSO film with nanocrystalline Si. Some processspecifics have been given to illustrate the method. However, theinvention is not limited to just these examples. Other variations andembodiments of the invention will occur to those skilled in the art.

1-17. (canceled)
 18. An electroluminescence (EL) device comprising: abottom substrate; a silicon-rich silicon oxide (SRSO) film including: afirst thickness overlying the substrate doped with a first concentrationof a rare earth (RE) element; a second thickness, overlying the firstthickness, doped with a second concentration of the RE element; and, atop electrode overlying the SRSO film.
 19. An electroluminescence (EL)device comprising: a bottom substrate; a silicon-rich silicon oxide(SRSO) film including: a first thickness overlying the substrate dopedwith a first rare earth (RE) element; a second thickness, overlying thefirst thickness, doped with a second RE element; and, a top electrodeoverlying the SRSO film.
 20. A silicon-rich silicon oxide (SRSO) filmcomprising: a first thickness doped with a first concentration of a rareearth (RE) element; and, a second thickness, overlying the firstthickness, doped with a second concentration of the RE element.
 21. Asilicon-rich silicon oxide (SRSO) film comprising: a first thicknessdoped with a first rare earth (RE) element; and, a second thickness,overlying the first thickness, doped with a second RE element.